In the past it has been found desireable to manufacture solar reflector troughs extending longitudinally in uniform cross section and having an interior concave surface formed in a parabolic configuration. Such reflector troughs are oriented to face the sun in order to receive solar radiation therefrom and reflect the sun's rays onto a solar energy absorption medium. Such a medium may assume the form of a conduit carrying circulating fluid. The conduit is located longitudinally along the trough at the parabolic focus. Reflected energy is thereby absorbed in the circulating fluid in the conduit and employed to advantage for a variety of purposes. The fluid may be used to carry thermal energy to create electricity, or may itself be distilled. Examples of the conventional manufacture and utilization of such reflectors are depicted in U.S. Pat. Nos. 1,946,184 and 3,959,056.
Problems have heretofore existed in the conventional manufacture of solar reflectors in that an adequate reflective surface has not heretofore been available which could be produced to economic advantage. An ideal reflector should be formed in a continuous concave focusing configuration with total reflectivity of sunlight and without surface irregularities that would cause difusion. Heretofore, several approaches have been attempted in an attempt to achieve a suitable reflector. Mirrors have been employed positioned in side by side relationship to approximate a parabolic surface. However, one such mirror construction involves the use of silvered or metal coated glass, in which there is a high degree of energy absorption. A great deal of solar energy is lost by energy absorption within the reflector itself. The degree of energy reflected to the energy absorption medium at the parabolic focus is thereby adversely effected.
An alternative parabolic reflective surface can be formed from highly polished sheets of metal, such as aluminum. Aluminum sheets can be formed into a parabolic configuration, but as with other forms of sheet metal, there is a tendency for a certain degree of waviness and surface undulations to persist despite careful contouring of the aluminum. Also, the necessary polishing of the aluminum is highly time consuming and expensive. The waviness in sheet aluminum is further aggravated by the weight of the metal sheet itself, which when stretched between supports, tends to sag. Stainless steel sheets suffer from the same deficiencies as polished aluminum sheets, and in addition are commercially unavailable in the requisite quality. In addition the thickness of sheet metal requires a much greater quantity of either aluminum or steel than is necessary with the technique of the present invention. Only a very small fraction of metal is required to produce the reflective coating of the present invention as contrasted with the use of a sheet metal reflector.
Mirrored plexiglass can be suitably formed in a parabolic configuration and is smooth enough to provide good reflectivity. However, the surface of plexiglass becomes degraded with continued atmospheric exposure and can not be adequately cleaned. Also, plastic, like glass, tends to absorb some of the solar energy within itself.
It is an object of the present invention to provide an improved reflective surface on a parabolic reflector for use in a solar collector at a reduced cost as compared with conventional techniques. The improved surface is provided by vacuum depositing an extremely thin coating of aluminum onto a parabolic support. The aluminum layer so deposited is preferably less than 0.001 inches in thickness so that only a small amount of solar energy is absorbed. Using a parabolic solar reflector formed according to the present invention, 92.0 percent of light energy received by the parabolic surfaced is reflected. Furthermore, vacuum depositing the aluminum layer may be performed much more economically than polishing a sheet aluminum surface.
An object of the invention in its preferred form is to provide a protective shielding for the reflective layer. This is achieved by vacuum depositing a thin layer of silcon dioxide or dipping the reflective layer in acrylic resin to create a protective coating on the thin aluminum coating previously vacuum deposited. This prevents degradation of the reflective properties of the parabolic reflecting surface which might otherwise result from exposure to the environment. By vacuum depositing the silicon dioxide, the protective layer can be made thin enough so as not to absorb energy to any significant degree.
A further object of the invention is to provide a supporting surface for the reflective aluminum layer which closely approaches perfect parabolic geometry. In conventional techniques of pouring or molding a substrate of plastic resin or other supporting material, curing of the resin or other substrate composition is accompanied by surface aberations which subsequently result in defusion of light received thereat. During curing, thermosetting plastic resin releases heat and tends to expand. With conventional molding and curing techniques, this leads to irregularities throughout the reflective surface. The effect, when viewed by microscope, is to create a diffuse surface or as "orange peel" surface, so called by reason of its microscopic resemblance to the texture of the outer surface of citrus fruit. Also both types of surface aberrations may be produced. A smooth surface is achieved according to the present invention by spraying a plurality of different layers of thermosetting resin onto an improved convex form in an number of different operations. Thus, curing of each thin sprayed layer proceeds to some extent before the application of the next layer. This improves the resultant surface conformity of the thermosetting resin sheet so formed to the exact parabolic configuration desired. Surface irregularities from expansion during curing are further reduced by utilizing fiberglass veils and chopped or shredded fiberglass roving. This reduces any tendency for the resin to run, and thereby vary in thickness on the convex form. A thickness variation in the resin affects the uniformity of rate of curing of the resin and creates surface irregularities as a result.
Surface irregularities due to expansion of the thermosetting resin in the support are further reduced by placing the edges of the convex mold under tension that tends to draw them toward each other. That is, the edges are pulled toward the principal axis of the parabolic configuration which aids in holding the parabolic shape of the reflector support as the fiberglass expands due to heat released during curing.
A further object of the invention is to provide an improved tool for forming parabolic solar reflectors. This is achieved by utilizing a convex mold in which a smooth surface film is supported on an underlying sheet arranged in a longitudinally extending configuration of parabolic cross section. Preferably, this smooth film is formed of plastic having a metallic chromium coating on the exterior surface thereof. The use of such a mold form enhances the exact conformity of the reflector surface to a configuration of precise parabolic cross section and minimizes diffusive irregularities in the reflective surface.